On their articulatory level vowels can change

3. The system of English vowels (20 phonemes)
Vowels unlike consonants are produced with no obstruction to the stream of air their integral characteristic is tone.

On their articulatory level vowels can change:

1. Stability of articulation.
Monophthongs (articul. Is unchanging), diphthongs(organs of speech glide from one position to another within 1 syllable. The starting point(nucl.) is strong $ distinct, glide is weak- ei, эi, au). Diphthongoids (artic. is slightly changing [i:], [u:].

2. The position of the tongue, another principle for consideration, is characterized from two aspects: horizontal and vertical movement. According to the horizontal movement five classes of vowels are distinguished:

According to the vertical movement three classes of vowels are distinguished:

3. Lip rounding is another articulatory feature which is included into the principles of classification of English vowels. Traditionally three lip positions are distinguished: spread, neutral and rounded. For the purpose of classification it is sufficient to distinguish between two lip positions: rounded and unrounded or neutral.

4. Another property of English vowel sounds is traditionally termed checkness (character of vowel end). The degree of checkness may vary and depends on the following consonants. Before voiceless consonant it is more perceptible than before a lenis voiced consonant or sonorant. All long vowels are free.

5. The English monophthongs are traditionally divided into two classes according to their length: a) short vowels: [I], [e], [;:e], [u], [A], [a], [n]; b) long vowels: [i:], [a:], [;):], [3:], [u:].

6. tenseness. It characterizes the state of the organs of speech at the moment of the production of vowels. Special instrumental analysis shows that historically long vowels are tense- i, e, u, o, ɔ, ɑ, ɝ while historically short vowels are lax- ɪ, ɛ, æ, ʊ, ə, ʌ, ɚ.

4. Comparison of English and Russian consonant system

a) in the production of the Russian consonants the bulk of the tongue is mainly in the front-mid part of the mouth resonator. When Russian soft forelinguals are produced the muscular tension is concentrated in the front-mid part of tongue, when the soft back lingual consonants are produced the muscular tension is concentrated in the middle part of the tongue. In the production of the English fore lingual consonants the tip of the tongue and the front edges are very tense;

b) the English voiceless fortis /p, t, k, f, s, ∫, t∫/ are pronounced more energetically then similar Russian consonants;

c) the bilabial [w] which is pronounced with a round narrowing is very often mispronounced by the Russian learners. They use the labio-dental [b] or [v] which is uttered with flat narrowing instead of the English [w];

d) there are definite consonant phonemes in English which have no counterparts in Russian, they are [w, Ө, ð, dg, r, h ].

5. Comparison of English and Russian vowel system
a) in the production of Russian vowels the lips are considerably protruded and rounded. In the articulation of the similar English sounds such protrusion doesn't take place. Englishmen have the so-called" flat-type" position of the lips which are more tense than the lips of the Russian and the comers of the lips are raised to resemble a smile;

b) in the articulation of the English vowels the bulk of the tongue occupies more flat positions than in the production of the Russian vowels;

c) long vowels in English are considered to be tense. There are no long vowels which can be opposed phonemically to short vowels in the Russian language. Length in the Russian vowel system is an irrelevant feature;

d) there are monophthongs and diphthongoids in the 'Russian vowel system, but there no diphthongs (diphthongized sounds, in English they are [i:] and [u:]

e) there are 6 vowel phonemes in Russian and 20 in English, the majority of the English vowel phonemes have no counterparts in Russian: [æ, 3:, ə, u: εə, uə, ai,əu ]